A typical electrochemical cell may include a cathode side and an anode side separated by a separator arrangement. In the redox flow battery art the cathode is the positive side, and the anode is the negative side. The positive cathode side may include a cathode current collector, a cathode electroactive material (reduced on discharge) and an electrolyte. The negative anode side may include an anode current collector, an anode electroactive material (oxidized on discharge) and an electrolyte. The cell separator separating the cathode and anode sides, inter alia, permits ionic flow therebetween. The current collectors, electroactive materials, electrolytes and cell separator thus form an electrochemical cell that converts chemical energy to electricity. Hence, the current collectors may be (externally) electrically connected together to form an electrical circuit.
In redox flow battery systems the electrodes are generally solid inert materials, and the electrolytes are flowing liquids that contain the electroactive materials. The electrolytes may be aqueous or non-aqueous solutions. The electrodes serve as sites where electrochemical reactions take place. The positive liquid is known as the catholyte, and the negative liquid is known as the anolyte.
The respective electrolytes are generally circulated through the respective sides of the cell by way of fluid circulation systems that are external to the cell. According to certain embodiments, each of the external catholyte and anolyte circulation systems includes an electrolyte reservoir. Some embodiments have an external circulation system on only one side of the cell. Several cells may share the same set of reservoirs, if desired. Systems in which only one of the electrolytes is circulated externally of the cell are generally described as partial redox flow battery systems.
Charging and discharging of the electrolytes generally take place in the reactive cell. The electrolytes are stored in their respective reservoirs outside of the cell. If desired, charging may be accomplished by replenishing the spent electrolytes in the respective reservoirs with fresh electrolytes.
The power output of the cell is generally determined by its physical size, and the capacity of the cell is determined by the size of the external electrolyte reservoirs.
Certain redox flow battery systems experience the formation of undesired precipitates in the catholyte. Such undesired precipitates form particularly during the charging phase in certain secondary redox flow battery systems. Numerous chemical reactions take place on both the cathode side and anode side of an electrochemical cell during the normal operation of a redox flow battery system. Many of these chemical reactions involve the reactants, or are catalyzed by trace elements in the respective electrolytes. Many of the reactants and reaction products are transitory, and do not significantly influence the performance of the system. Where a solid precipitate is formed that is of such a nature that it impairs the operation of the system, it is a problem that needs to be solved.